This invention is related to catalytic cracking of hydrocarbons. It more particularly refers to improvements in the endothermic catalytic cracking of petroleum fractions and alternative exothermic catalyst regeneration.
Endothermic catalytic cracking of hydrocarbons, particularly petroleum fractions, to lower molecular weight desirable products is well known. This process if practiced industrially in a cycling mode wherein hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst without added hydrogen at rather low pressures of up to about 50 psig and temperatures sufficient to support the desired cracking. As the hydrocarbon feed is cracked to lower molecular weight, more valuable and desirable products, "coke" is deposited on the catalyst particles. The coked catalyst is disengaged from the hydrocarbon products, which are then separated into appropriate components. The coked catalyst particles, now cooled from the endothermic cracking and disengaged from the hydrocarbon products, are then contacted with an oxygen containing gas whereupon coke is burned off the particles to regenerate their catalytic activity. During regeneration, the catalyst particles absorb the major portion of the heat generated by the combustion of coke, i.e., they are "reflexively" heated, with consequent increase of catalyst temperature. The heated, regenerated catalyst particles are then contacted with additional hydrocarbon feed and the cycle repeats itself.
A flue gas comprising carbon oxides and, to a lesser extent, nitrogen oxides is produced during regeneration. The carbon monoxide and nitrogen oxides are sometime vented to the atmosphere with the rest of the flue gas. Alternatively, the carbon monoxide in some plants is burned to carbon dioxide, in a CO boiler to recover process steam and reduce emissions.
Two major variants for endothermically cracking hydrocarbons are fluid catalytic cracking (FCC) and moving bed catalytic cracking. In both of these processes as commercially practiced, the feed hydrocarbon and the catalyst are passed through a "reactor"; are disengaged; the catalyst is regenerated with cocurrent and/or countercurrent air; and the regenerated reflexively heated catalyst recontacted with more feed to start the cycle again. These two processes differ substantially in the size of the catalyst particles utilized in each and also in the engineering of materials contact and transfer which is at least partially a function of the catalyst size.
In fluid catalytic cracking (FCC), the catalyst is a fine powder of about 10 to 200 microns, preferably about 70 micron, size. This fine powder is generally propelled upwardly through a riser reaction zone suspended in and thoroughly mixed with hydrocarbon feed. The coked catalyst particles are separated from the cracked hydrocarbon products, and after purging are transferred into the regenerator where coke is burned to reactivate the catalyst. Regenerated catalyst generally flows downward from the regenerator to the base of the riser.
One typical example of industrially practiced moving bed hydrocarbon catalytic cracking is known as Thermofor Catalytic Crackig (TCC). In this process the catalyst is in the shape of beads or pellets having an average particle size of about 1/64 to 1/2 inch, preferably about 1/8 inch. Active, hot catalyst beads progress downwardly cocurrent with a hydrocarbon charge stock through a cracking reaction zone. In this zone hydrocarbon feed is endothermically cracked to lower molecular weight hydrocarbons while coke is deposited on the catalyst. At the lower end of the reaction zone the hydrocarbon products are separated from the coked catalyst, and recovered. The coked catalyst is then passed downwardly to a regeneration zone, into which air is fed such that part of the air passes upwardly countercurrent to the coked catalyst and part of the air passes downwardly cocurrent with partially regenerated catalyst. Two flue gases comprising carbon oxides are produced. Regenerated catalyst is disengaged from the flue gas and is then lifted, pneumatically or mechanically, back up to the top of the reaction zone.
The catalysts used in endothermic catalytic nonhydrogenative cracking are to be distinguished from catalysts used in exothermic catalytic hydrocracking. Operating conditions are also to be distinguished. While the catalytic cracking processes to which this invention is directed operate at low pressures near atmospheric and in the absence of added hydrogen, hydrocracking is operated with added hydrogen at high pressures of up to about 1000 to 3000 psig. Further, nonhydrogenative catalytic cracking is a reflexive process with catalyst cycling between cracking and regeneration (coke burn off) over a very short period of time, seconds or minutes. In hydrocracking, on the other hand, the catalyst remains in cracking service for an extended period of time, months, between regeneration (coke burn off). Another important difference is in the product. Nonhydrogenative catalytic cracking produces a highly unsaturated product with substantial quantities of olefins and aromatics, and a high octan gasoline fraction. Hydrocracking, in contrast, produces an essentially olefin-free product with a relatively low octane gasoline. This invention is not directed to hydrocracking nor is it within the scope of this invention to use hydrocracking catalysts in the process hereof.
The invention is directed to catalytic cracking units, such as the FCC processes shown in U.S. Pat. No. 3,904,548 (swirl type regeneration), U.S. Pat. No. 3,886,060 (catalytic cracking of resids), U.S. Pat. No. 4,654,060 (closed cyclones on riser reactor outlet). The teachings of all of these patents are incorporated by reference.
The invention uses conventional FCC catalysts. The last two decades have seen major developments in catalyst compositions for use in catalytic cracking. The catalysts formerly widely employed in FCC and TCC have included acid treated clays, amorphous silica-alumina composites and the like. Many variants, such as silica-zirconia, silica-magnesia and other acidic porous solids have been described in the literature
The first major development provided much more effective catalysts by blending a major portion of the older amorphous catalysts with a minor portion of an active crystalline aluminosilicate zeolite. Catalysts of this type for FCC and TCC are described in U.S. Pat. Nos. 3,140,249 and 3,140,253, which are hereby incorporated by reference.
Another major development was CO combustion in the regenerator. Many FCC units now operate with trace amounts of a platinum group metal, such as 1.0 parts ppm Pt, incorporated with the cracking catalyst. The Pt catalyzes the complete burning of carbon monoxide to carbon dioxide in the regenerator without detriment to the cracking reaction. This development has been very widely accepted by refiners. Such catalysts and their use are described in U.S. Pat. Nos. 4,251,395; 4,265,787; 4,088,568; 4,072,600; 4,093,535 and 4,159,239; all to Schwartz, are incorporated herein by reference. Such promoted catalysts may be used to completely burn CO (referred to hereinbelow as "full CO combustion") or to only partially burn the CO (referred to hereinbelow as "partial CO-combustion") by the simple expedient of limiting the oxygen supplied to the regenerator. The term "Pt group metal CO-oxidation promoter" as used herein means those metals and their mode of use as taught by the Schwartz patents cited above. Full CO combustion exacerbates some pre-existing problems (metal contamination) and creates some new problems (NO.sub.x, SO.sub.x emissions). The more active zeolite catalysts allowed refiners to process heavier charge stocks, and include minor amounts of resid in the feed. Metals levels on catalyst increased.